1. Field of the Invention
This invention relates to an image reading apparatus that reads an image by using an image sensor with light receivers aligned one dimensionally.
2. Background Art
An image reading apparatus that reads out an image from an original by using an image sensor with one-dimensionally-aligned light receivers has been known. This type of image reading apparatus is generally installed in a photo copier, facsimile, scanner, or a multifunction machine wherein functions of these devices are combined in one machine.
For an image sensor an image sensor with CCD (Charge Coupled Devices) method, or CIS (Contact Image Sensor) are, for example, known. Amongst theses image sensors, especially CIS are often used, since they does not require complex optical systems for size reduction of a subject image while this system is necessary for an image sensor with CCD method to read out an image, and consequently size of CIS can be considerably smaller and lighter than size of an image sensor with CCD method.
Following constitutions are generally known as specific structures of CIS. One is a single output type image, sensor (CIS) which is constituted with a single reading device wherein light receivers are one dimensionally aligned. In this single output type image sensor, a length in main scanning direction of the single reading device (a same direction as an alignment direction of the light receivers) is a length the image sensor can read in one line of an image in the main scanning direction. Pixel signals read out by the single reading device are sequentially outputted by this single reading device.
Another constitution is a parallel output type image sensor (CIS) which is constituted with plurality of reading devices with one dimensionally aligned light receivers provided in a main scanning direction of an original. In this parallel output type image sensor, one line of an image in the main scanning direction is divided by the reading devices and respectively read out. Pixel signals read out by reading devices are respectively outputted in parallel. It is possible, with this type of parallel output type image sensor, to make reading time for reading out one line of an image shorter, compared to reading time with the single output type image sensor, as disclosed in Unexamined Japanese Patent Publication No.9-130592.
For example, in CIS wherein two reading devices are adjacently disposed so that light receivers of each reading device are aligned one dimensionally in a main scanning direction, data read out from left half of an original and data read out from right half of the original can be simultaneously outputted. Consequently, reading an original can be conducted fast. However, since the data from the left half and the right half of an original are respectively outputted form each reading device, positional relation of pixel signals sequentially obtained from these reading devices are not consecutive.
In order to match the pixel signals sequentially read in a consecutive state with one dimensional alignment order in the main scanning direction of respective light receivers constituting an image sensor (to be referred to as alignment order in the main scanning direction), JP 9-130592 discloses that all obtained pixel data are once stored into a memory, and read out in the main scanning direction by controlling reading address.
For image reduction process (or low resolution process), pixel skipping is generally conducted in specific cycle so that an image is reduced by a target rate. JP 9-130592 discloses that pixel data can be read out while being skipped in a cycle corresponding to a rate of reduction by controlling an address during reading.